The ability to selectably vary the temperature of a sample during NMR analysis is a known technique for the enhancement of NMR signal characteristics and resulting spectra. Apparatus to accommodate selectable temperature NMR analysis is known in the prior art, frequently in the form of a dewar constructed for disposition within the NMR probe, and thus subject to severe spatial constraints for location between the poles of a conventional magnet or in the bore of a superconducting magnet. Such apparatus necessarily occupies a substantial portion of the analysis volume, thereby reducing the volume available for the sample. Consequently, dewar-based systems are at a disadvantage in respect to sensitivity by virtue of a reduced sample volume and separately, are characterized by degraded resolution because the larger volume required penalizes the achievable field homogeneity. The latter problem is subject to remedy by imposition of controllable gradient fields carefully designed to cancel out residual inhomogeneities. However, the gradient corrections are achieved with coils which are necessarily disposed as close as possible to the sample region. To the extent that heat developed or absorbed in the sample volume may affect the surrounding gradient correction coils, the gradient correction suffers and resolution is reduced.
Dewar systems suffered further disadvantages. The dimensional contraints result in a somewhat fragile component. Mechanical handling of the probe assembly and disassembly of portions of the probe are routine operations. Mechanical spinning and sample ejection are also sources of possible mechanical malfunction wherein the dewar may suffer damage. Dewar integrity includes the vacuum integrity of the dewar vessel. It is apparent that the dewar is a non-trivial expense item in ordinary operation.
The dewar is also subject to RF fields which may induce electrical breakdown within the dewar vacuum vessel resulting in substantial further RF noise detrimental to the resonance signal processing.